Complete ablation of nonmuscle myosin heavy chain II-B (NMHC-B) in mice resulted in cardiac and brain defects that were lethal during embryonic development or on the day of birth (Tullio et al., Proc. Natl. Acad. Sci. USA 94, 12407-12412, 1997). We have now generated B-delta-I/B- delta-I hypomorphic mice by replacing a neural-specific alternative exon with the PGK-Neo cassette and this resulted in decreased amounts of NMHC-B in all tissues. B-delta-I/B-delta-I mice developed cardiac myocyte hypertrophy between 7 and 11 months of age and approximately 4% of these mice developed overt signs of hydrocephalus. Serial sections of B-delta-I/B-delta-I brains showed abnormalities in neural cell migration and adhesion in the ventricular wall. Crossing B-delta-I/B- delta-I with B+/B- mice generated B-delta-I/B- mice, which showed a further decrease in NMHC-B. Five of eight B-delta-I/B- mice were born with a membranous ventricular septal defect and these mice developed myocyte hypertrophy by 1 month. Moreover, over 60% of them developed overt hydrocephalus and showed more severe defects in neural cell migration and adhesion. These data demonstrate a gene dosage effect between the amount of NMHC-B and the severity and time of onset of the defects in the heart and brain. - nonmuscle myosin; hydrocephalus; ventricular septal defect; myosin heavy chain